1. Field of the Invention
The present invention relates to an image capture apparatus and a method of controlling the same, and in particular relates to a focus detection technique.
2. Description of the Related Art
In image capture apparatuses that use an image sensor, typified by a digital camera, automatic focus detection (AF) is realized by methods mainly divided into a method of using a device for focus defection that is separate from the image sensor, and a method of using the image sensor. The method of using a device for focus detection is a method of obtaining a defocus amount using a correlation amount (phase difference) between two types of image waveforms generated from light flux that exited different exit pupils of an imaging optics system, which is called a phase-difference detection method. Also, the method of using the image sensor is a method of, using the fact that the contrast component of a captured image is the highest at the in-focus position, searching for the focusing lens position at which the contrast component is the highest, which is called a contrast detection method.
The phase-difference detection method has an advantage in that since the defocus amount is immediately obtained from the correlation amount between the image waveforms, only a short amount of time is needed to move the focusing lens to the in-focus position, but on the other hand, there are the problems of an increase in the size of the image capture apparatus and an increase in cost due to the need to provide the device for focus detection. Also, since subject images need to be captured by the device for focus detection during focus detection, focus detection cannot be performed in parallel with imaging performed by the image sensor, or it is necessary to provide a special mechanism such as a half mirror.
For this reason, Japanese Patent Laid-Open No. 2010-152161 proposes a configuration for realizing automatic focus detection using the phase-difference detection method in which light flux that has passed through different exit pupils of the imaging optics system is received by pixels of the image sensor (phase-difference detection on an imaging surface method, which will be referred to hereinafter as “phase-detection AF on an imaging surface”). Phase-detection AF on an imaging surface is characterized by not requiring a device for focus detection and also making it possible for automatic focus detection using the phase-difference detection method (phase difference AF) to be carried out in parallel with imaging performed by the image sensor.
It is desirable for the image signals necessary for phase difference detection in phase-detection AF on an imaging surface to be generated from signals obtained from a two-dimensional (multiple pixel rows×multiple pixel columns) pixel region (focus detection region). This is because it is difficult to precisely perform phase difference detection using image signals generated from signals obtained from a one-dimensional focus detection region (one pixel row or column).
Japanese Patent Laid-Open No. 2010-152161 discloses that image signals are generated from pixels, a pair of focus detection image signals are generated by aligning the signals in each pixel row in the direction in which the pixel columns extend, and the phase difference (image misalignment amount) is calculated from the one-dimensional focus detection image signals. It also discloses a method of obtaining a two-dimensional focus detection image misalignment amount by adding up the image misalignment amounts of multiple pixel lines from a two-dimensional focus detection region.
In general, when performing imaging using phase-detection AF on an imaging surface, focus detection is performed while capturing moving images using the image sensor, and the captured video is displayed as a live preview on the display unit of the image capture apparatus. The pixel count of image sensors currently in common use is greater than the pixel count of the display unit, and therefore the video to be displayed as a live preview is generated by thinning out the pixels in the captured video frames. Also, thinning out the pixels while performing readout from the image sensor reduces the processing load compared to thinning out the pixels after reading all of them out. Furthermore, the thinning number (thinning rate) is not constant, but rather is generally increased as the load increases (e.g., as the frame rate increases). After thinning-readout, resize processing is performed as necessary in order to obtain a display image.
However, in the image signal generation method described in Japanese Patent Laid-Open No. 2010-152161, when a pixel signal is thinned out, the pixel range of the image sensor in which focus detection image signal readout is performed changes according to the thinning number. Since the thinning number changes according to the readout frame rate, for example, in the case of using different frame rates for different shooting modes, the focus detection image signal 0
Letting m be the thinning number, a pixel range having n lines in the thinned image is constituted by pixels read out from a pixel range of m×n lines in the pre-thinning image (original image). Accordingly, the lower the thinning number m is, the smaller the corresponding pixel area in the original image is. Therefore, the area of the original image used to acquire the image signal (readout area) is smaller when the thinning number is low than when the thinning number is high, and the entirety of the focus detection region cannot be covered, and thus the precision of the focus detection decreases.